Biomedical Engineering Reference
In-Depth Information
(a)
(b)
2.4
Instrumentation for (a) WAXS and (b) SAXS. (Source: Images
courtesy of Bruker AXS Inc.)
2.3.1 Crystal structure analysis
Atoms in metals and ceramics are almost always arranged into regular lat-
tices. Polymer molecules too can be arranged into similar lattices. At the
smallest length scales, established crystallographic techniques are used to
determine the crystal structures from which the atomic coordinates can be
derived. Ideally, single crystals are a prerequisite to determine the crystal
structure. However, numerous structures have been solved using data from
fi bers and polycrystalline powders. These techniques are very relevant to
biomaterials that cannot always be obtained as single crystals. An example
of the arrangement of atoms that can be derived from such crystal structure
analysis from fi ber-diffraction data is shown in Fig. 2.5.
4
This structure is for
poly(ethylene oxide), which is widely used as co-macromer in biomaterials
to increase the hydrophilicity of a polymer. The structure shows that PEO
is essentially a 7/2 helix (seven monomer units in two turns) consisting of
TTG sequences. The crystal structures are essential for understanding the
interactions between a polymer and a substrate, and between a polymer and
a host molecule in, for example, drug delivery devices.
2.3.2 Phase identifi cation
XRD is often used to identify and determine the fractions of different
phases that are present in a sample. For instance in Fig. 2.6, the XRD pat-
tern of the sample can be used to identify the form of calcium phosphate
in the sample along with other phases that could be present in the calci-
fi ed deposits of an implanted ostrich pericardium.
5
The three scans for the
implanted pericardium show an amorphous peak due to collagen at 2
θ
~ 10°
along with a second amorphous halo at 2
θ
~30° due to an amorphous form
of apatite (calcium phosphate). None of the scans show the crystalline form
